The search for polyethylene products having an improved balance of physical properties and processability has led to the development of products having improved output capacity and ever improving end use properties such as enhanced film tear or dart impact properties. Particularly useful is the development of polymer architectures for which polymer blending strategies can be avoided for enhancement of polymer properties, since these strategies increase cost.
U.S. Pat. Appl. No. 2011/0003099 discusses low melt flow ratio (MFR) linear polyethylene and high melt flow ratio (MFR) linear polyethylene, which are distinguished by an I21/I2 of less than 30 and an I21/I2 of greater than 30 respectively.
Resins having both a narrow molecular weight distribution and a low melt flow ratio are well known and include resins produced with metallocene catalysts and phosphinimine catalysts. Such resins include for example Exceed 1018A™ from ExxonMobil and those described in U.S. Pat. No. 5,420,220 and Canadian Pat. Appl. No. 2,734,167. These resins can be made into films having a good balance of physical and optical properties, but can be difficult to process in the absence of processing aids, as indicated by, for example, a relatively low output capacity on a blown film line.
Resins having a higher melt flow ratio are more attractive to film producers because they are generally easier to process. U.S. Pat. Nos. 6,255,426 and 6,476,171 and U.S. Pat. Appl. No. 2011/0003099 each describe the production and use of resins having melt flow ratios which are in excess of 30 and which have moderately broad molecular weight distributions. The resins are thought to contain long chain branching. The polymers disclosed in U.S. Pat. Nos. 6,255,426 and 6,476,171 are made with a bridged bis-indenyl zirconocene catalyst and have a composition distribution breadth index (CDBI) of greater than 75%. The resins have been referred to as Enable™ polymers (ExxonMobil) in the patent literature (see for example, the Example Polymers disclosed in U.S. Pat. Appl. No. 2011/0003099), and although the resins are relatively easy to process, they also have a good balance of strength and stiffness properties when blown into film. For example, the films had physical properties which were comparable to Exceed 1018A materials despite their better shear thinning behavior. The polymers disclosed in U.S. Pat. Appl. No. 2011/0003099, include a new “Enable” grade resin having a low melt index (I2=0.3), a relatively high melt flow ratio (I21/I2 is from 46-58) and a moderately broad molecular weight distribution (e.g. Mw/Mn is 3.4). The polymers also have a single peak in a TREF profile, with a T(75)-T(25) of less than 4° C.
Manipulation of the comonomer distribution profile has also provided novel ethylene copolymer architectures in an effort to improve the balance between physical properties and polymer processability.
It is generally the case that metallocene catalysts and other so called “single site catalysts” typically incorporate comonomer more evenly than traditional Ziegler-Natta catalysts when used for catalytic ethylene copolymerization with alpha olefins. This fact is often demonstrated by measuring the composition distribution breadth index (CDBI) for corresponding ethylene copolymers. The definition of composition distribution breadth index (CDBI50) can be found in PCT publication WO 93/03093 and in U.S. Pat. No. 5,206,075. The CDBI50 is conveniently determined using techniques which isolate polymer fractions based on their solubility (and hence their comonomer content). For example, temperature rising elution fractionation (TREF) as described by Wild et al. J. Poly. Sci., Poly. Phys. Ed. Vol. 20, p 441, 1982 can be employed. From the weight fraction versus composition distribution curve, the CDBI50 is determined by establishing the weight percentage of a copolymer sample that has a comonomer content within 50% of the median comonomer content on each side of the median. Generally, Ziegler-Natta catalysts produce ethylene copolymers with a CDBI50 lower than that of a single site catalyst at a similar density consistent with a heterogeneously branched copolymer. Typically, a plurality of prominent peaks is observed for such polymers in a TREF (temperature raising elution fractionation) analysis. Such peaks are consistent with the presence of heterogeneously branched material which generally includes a highly branched fraction, a medium branched fraction and a higher density fraction having little or no short chain branching. In contrast, metallocenes and other single site catalysts, will most often produce ethylene copolymers having a CDBI50 higher than that of a Ziegler-Natta catalyst at similar density and which often contain a single prominent peak in a TREF analysis, consistent with a homogeneously branched copolymer.
Despite the forgoing, methods have been developed to access polyethylene copolymer compositions having a broadened comonomer distribution (i.e. more Ziegler-Natta like) while otherwise maintaining product characteristics typical of metallocene and single site catalyst resin, such as high dart impact strength for blown film. Such resins can be made, for example, by using a mixture of metallocene catalysts in a single reactor, using a plurality of polymerization reactors under different polymerization conditions, or by blending metallocene produced ethylene copolymers.
U.S. Pat. Nos. 5,382,630, 5,382,631 and WO 93/03093 describe polyethylene copolymer blend compositions having broad or narrow molecular weight distributions, and broad or narrow comonomer distributions. For example a blend may have a narrow molecular weight distribution, while simultaneously having a bimodal composition distribution. Alternatively a blend may have a broad molecular weight distribution while simultaneously having a unimodal composition distribution. The blends are made by melt blending two polyethylene resins with similar or different molecular weights and similar or different comonomer contents, where each resin is formed using a metallocene catalyst in a gas phase reactor.
U.S. Pat. No. 7,018,710 discloses blends comprising a high molecular weight component having a high comonomer content and a low molecular weight component having a low comonomer content. The ethylene copolymer blend, which arises from the use of a metallocene catalyst in a cascade dual reactor process where each reactor is operated under different conditions (e.g. a cascade slurry phase-gas phase reactor), shows two distinct maxima in a TREF fractogram. The polymers were applied as a sealing layer in a heat sealable film.
A mixed catalyst system containing a “poor comonomer incorporator” and a “good comonomer incorporator” is disclosed in U.S. Pat. Nos. 6,828,394 and 7,141,632. The poor comonomer incorporating catalyst may be a metallocene having at least one fused ring cyclopentadienyl ligand, such as an indenyl ligand, with appropriate substitution (e.g. alkyl substitution at the 1-position). The good comonomer incorporating catalyst was selected from an array of well-known metallocenes and which was generally less sterically encumbered toward the front end of the molecule than the poor comonomer incorporator. These mixed catalyst systems produced polyethylene copolymers having a bimodal TREF distribution in which two elution peaks are well separated from one another, consistent with the presence of higher and lower density components. The mixed catalysts also produced ethylene copolymer having a broadened molecular weight distribution relative to ethylene copolymer made with either one of the single metallocene component catalysts.
A mixed catalyst system comprising three distinct metallocene catalysts is disclosed in U.S. Pat. No. 6,384,158. Ethylene copolymers having broadened molecular weight distributions were obtained when using these catalyst systems to polymerize ethylene with an alpha olefin such as 1-hexene.
U.S. Pat Appl. No. 2011/0212315 describes a linear ethylene copolymer having a bimodal or multimodal comonomer distribution profile as measured using DSC, TREF or CRYSTAF techniques. The copolymers maintain a high dart impact resistance when blown into film and are relatively easy to process as indicated by a reduced shear thinning index, relative to ethylene copolymers having a unimodal comonomer distribution profile. The exemplified ethylene copolymer compositions, which have a melt flow ratio of less than 30, are made in a single gas phase reactor by employing a mixed catalyst system comprising a metallocene catalyst and a late transition metal catalyst.
U.S. Pat. No. 7,534,847 demonstrates that use of a chromium based transition metal catalyst gives an ethylene copolymer having a bimodal comonomer distribution (as indicated by CRYSTAF) with a CDBI of less than 50 wt % (see Table 1 of U.S. Pat. No. 7,534,847). The patent teaches that the copolymers may have a molecular weight distribution of from 1 to 8, significant amounts of vinyl group unsaturation, long chain branching and specific amounts of methyl groups as measured by CRYSTAF fractionation.
U.S. Pat. No. 6,932,592 describes very low density (i.e. <0.916 g/cc) ethylene copolymers produced with a bulky non-bridged bis-Cp metallocene catalyst. A preferred metallocene is bis(1-methyl-3-n-butylcyclopentadienyl)zirconium dichloride. The examples show that in the gas phase, supported versions of this catalyst produce copolymer from ethylene and 1-hexene which has a CDBI of between 60 and 70% and a bimodal comonomer distribution as measured by temperature raising elution fractionation (TREF).
U.S. Pat. No. 6,420,507 describes a low density ethylene copolymer having a narrow molecular weight distribution (i.e. 1.5 to 3.0) and a bimodal TREF profile. The polymerization is carried out in the gas phase using a so called “constrained geometry” catalyst having an indenyl ligand.
U.S. Pat. Nos. 6,248,845, 6,528,597, 7,381,783 and U.S. Pat. Appl. No. 2008/0108768 disclose that a bulky ligand metallocene based on hafnium and a small amount of zirconium can be used to provide an ethylene/1-hexene copolymer which has a bimodal TREF profile. It is taught that the hafnium chloride precursor compounds used to synthesize the bulky metallocene catalysts are either contaminated with small amount of zirconium chloride or that zirconium chloride may be deliberately added. The amounts of zirconium chloride present range from 0.1 mol % to 5 mol %. Hence, the final hafnocene catalysts contain small amounts (i.e. 0.1 to 5 mol %) of their zirconocene analogues. Since zirconium based catalysts can have superior activity relative to their hafnium analogs it is possible that the products made have a significant contribution from the zirconocene species. If this is the case, then it is perhaps not surprising that a bimodal TREF profile results. The patent provides data for cast and blown film applications which shows that compared to Exceed type resins, the polymers are more easily extruded, with lower motor load, higher throughput and reduced head pressure. The resins give cast film with high tear values and blown film with high dart impact values.
U.S. Pat. Nos. 6,956,088, 6,936,675, 7,179,876 and 7,172,816 disclose that use of a “substantially single” bulky ligand hafnium catalyst provides an ethylene copolymer composition having a CDBI of below 55%, especially below 45% as determined by CRYSTAF. Recall, that hafnocene catalysts derived from hafnium chloride are expected to have zirconocene contaminants present in low amounts. U.S. Pat. Nos. 6,936,675 and 7,179,876 further teach that the CDBI could be changed under different temperature conditions when using hafnocene catalysts. Polymerization at lower temperatures gave ethylene copolymer having a broader composition distribution breadth index (CDBI) relative to polymers obtained at higher temperatures. For example, use of the catalysts bis(n-propylcyclopentadienyl)hafnium dichloride or bis(n-propylcyclopentadienyl)hafnium difluoride in a gas phase reactor for the copolymerization of ethylene and 1-hexene at ≦80° C., gave copolymers having a CDBI of between 20 and 35%, compared to CDBI values of between 40 and 50% for copolymers obtained at 85° C. The polymers disclosed can, under certain draw down ratios, provide films having a machine direction tear value of greater than 500 g/mil, a dart impact resistance of greater than 500 g/mil, as well as good stiffness. The polymers also have good processability.
U.S. Pat. No. 5,281,679 describes bis-cyclopentadienyl metallocene catalysts which have secondary or tertiary carbon substituents on a cylcopentadienyl ring. The catalysts provide polyethylene materials with broadened molecular weight during gas phase polymerization.
Cyclic bridged bulky ligand metallocene catalysts are described in U.S. Pat. Nos. 6,339,134 and 6,388,115 which give easier processing ethylene polymers.
A hafnocene catalyst is used in U.S. Pat. No. 7,875,690 to give an ethylene copolymer in a gas phase fluidlzed bed reactor. The copolymer has a so called “broad orthogonal composition distribution” which imparts improved physical properties and low extractables. A broad orthogonal composition distribution is one in which the comonomer is incorporated predominantly in the high molecular weight chains. The copolymers had a density of at least 0.927 g/cc. Polyethylene copolymers having a similarly broad orthogonal composition distribution but a lower density are disclosed in U.S. Pat. No. 8,084,560 and U.S. Pat. Appl. No. 2011/0040041A1. Again a hafnocene catalyst is employed in a gas phase reactor to give the ethylene copolymer.
U.S. Pat. No. 5,525,689 also discloses the use of a hafnium based metallocene catalyst for use in olefin polymerization. The polymers had a ratio of I10/I2 of from 8 to 50, a density of from 0.85 to 0.92 g/cc, a Mw/Mn of up to 4.0, and were made in the gas phase.
U.S. Pat. No. 8,114,946 discloses ethylene copolymers which have a molecular weight distribution (Mw/Mn) ranging from 3.36 to 4.29, a reversed comonomer incorporation and contain low levels of long chain branching. The melt flow ratios of the disclosed polymers are generally below about 30. A bridged cyclopentadienyl/fluorenyl metallocene catalyst having an unsaturated pendant group is used to make the ethylene copolymers. The patent application does not mention films or film properties.
U.S. Pat. No. 6,469,103 discusses ethylene copolymer compositions comprising a first and a second ethylene copolymer component. The individual components are defined using ATREF-DV analytical methods which show a bimodal or multimodal structure with respect to comonomer placement. The compositions have an I10/I2 value of greater 6.6 and a relatively narrow molecular weight distribution (i.e. Mw/Mn is less than or equal to 3.3) consistent with the presence of long chain branching. The polymers are made using a dual solution reactor system with mixed catalysts.
A process for making ethylene polymer compositions involving the use of at least two polymerization reactors is described in U.S. Pat. No. 6,319,989. The ethylene copolymers have a molecular weight distribution of greater than 4.0 and show two peaks when subjected to a crystallization analysis fractionation (CRYSTAF).
U.S. Pat. No. 6,462,161 describes the use of either a constrained geometry type catalyst or a bridged bis-Cp metallocene catalyst to produce, in a single reactor, a polyolefin composition having long chain branching and a molecular weight maximum occurring in the part of the composition having the highest comonomer content (i.e. a reversed comonomer distribution). The compositions made with a constrained geometry catalyst have multimodal TREF profiles, and relatively narrow molecular weight distributions (e.g. the exemplified resins have a Mw/Mn of from 2.19 to 3.4, see Table 1 in the examples section of U.S. Pat. No. 6,462,161). The compositions made with a bridged bis-Cp metallocene catalyst have complex TREF profiles and somewhat broader molecular weight distribution (e.g. the exemplified reins have a Mw/Mn of 3.43 or 6.0, see Table 1 in the Examples section of U.S. Pat. No. 6,462,161).
Ethylene copolymers are taught in U.S. Pat. No. 7,968,659 which have a melt index of from 1.0 to 2.5, a Mw/Mn of from 3.5 to 4.5, a melt elastic modulus G′(G″=500 Pa) of from 40 to 150 Pa and an activation energy of flow (Ea) in the range of 28 to 45 kJ/mol. Constrained geometry catalysts are used to make the polymer compositions in the gas phase.
U.S. Pat. No. 7,521,518 describes the use of a constrained geometry catalyst to give an ethylene copolymer composition having a reversed comonomer distribution as determined by various cross fractionation chromatography (CFC) parameters and a molecular weight distribution of from 2 to 10.
U.S. Pat. No. 5,874,513 describes that the use of a mixture of components which give rise to a supported metallocene catalyst can, in a gas phase reactor, give an ethylene copolymer with reduced comonomer distribution homogeneity. The patent defines a composition distribution parameter Cb which is representative of the distribution of comonomers within the polymer composition. The TREF analysis of the copolymer composition showed a bimodal distribution.
U.S. Pat. No. 6,441,116 discloses a film comprising an ethylene copolymer with a composition distribution curve obtained by TREF having four distinct areas including one peak defining area which is attributed to a highly branched component.
An ethylene/alpha olefin copolymer produced with a Ziegler-Natta catalyst and having greater than about 17 weight percent of a high density fraction, as determined by analytical TREF methods, and a molecular weight distribution (Mw/Mn) of less than about 3.6 is disclosed in U.S. Pat. No. 5,487,938. The high density fraction has little short chain branching, while the balance of the copolymer composition is referred to as the fraction containing short chain branching. Hence, the data is consistent with a bimodal distribution of comonomer incorporation into the ethylene copolymer.
U.S. Pat. No. 6,642,340 describes an ethylene copolymer having a specific relationship between a melt flow rate and melt tension. The polymers further comprise between 0.5 and 8 wt % of a component eluting at not lower than 100° C. in a TREF analysis.
Use of phosphinimine catalysts for gas phase olefin polymerization is the subject matter of U.S. Pat. No. 5,965,677. The phosphinimine catalyst is an organometallic compound having a phosphinimine ligand, a cyclopentadienyl type ligand and two activatable ligands, and which is supported on a suitable particulate support such as silica. The exemplified catalysts had the formula CpTi(N═P(tBu)3)X2 where X was Cl, Me or Cl and —O-(2,6-iPr—C6H3).
In co-pending CA Pat. Appl. No. 2,734,167 we showed that suitably substituted phosphinimine catalysts gave narrow molecular weight distribution copolymers which when made into film showed a good balance of optical and physical properties.
Polymers and films made in the gas phase using various single site catalysts, including so called “phosphinimine” catalysts, were disclosed at Advances in Polyolefins II, Napa, California—Oct. 24-27, 1999 (“Development of NOVA's Single Site Catalyst Technology for use in the Gas Phase Process”—I. Coulter; D. Jeremic; A. Kazakov; I. McKay).
In a disclosure made at the 2002 Canadian Society for Chemistry Conference (“Cyclopentadienyl Phosphinimine Titanium Catalysts—Structure, Activity and Product Relationships in Heterogeneous Olefin Polymerization.” R. P. Spence; I. McKay; C. Carter; L. Koch; D. Jeremic; J. Muir; A. Kazakov. NOVA Research and Technology Center, CIC, 2002), it was shown that phosphinimine catalysts bearing variously substituted cyclopentadienyl and indenyl ligands were active toward the gas phase polymerization of ethylene when in supported form.
U.S. Pat. Appl. No. 2008/0045406, discloses a supported phosphinimine catalyst comprising a C6F5 substituted indenyl ligand. The catalyst was activated with an ionic activator having an active proton for use in the polymerization of ethylene with 1-hexene.
U.S. Pat. Appl. No. 2006/0122054 discloses the use of a dual catalyst formulation one component of which is a phosphinimine catalyst having an n-butyl substituted indenyl ligand. The patent is directed to the formation of bimodal resins suitable for application in pipe.